1. Field of the Invention
The present invention relates to methods and apparatus for sensing environment hazards (e.g., a toxic gas). More particularly, this invention pertains to such hazard detection with a suppression circuit for inhibiting (i.e., reducing or eliminating) stray electrical current from associated equipment in proximity to the sensor to mitigate sensing errors.
2. Description of the Prior Art
As will be described, the present invention is preferably used for sensing environmental hazards in ambient air. For example, the present invention senses for toxic gases.
Toxic gases are a common risk associated with oil well drilling. Notably, such drilling can result in release of hydrogen sulfide, which is highly toxic. As a result, drill sites will be provided with one or more sensors for sensing the presence and concentration of hydrogen sulfide in ambient air.
Common prior art sensors for sensing hydrogen sulfide are so-called electro-chemical sensors. While such electrochemical sensors enjoy a reputation for accuracy, they are not without operational limitations. For example, such electro-chemical sensors are sensitive to environmental conditions. Namely, in hot and dry environments (common to many oil drilling facilities in arid locations), the sensors may dry out and become non-functional. In extremely cold environments, the sensors may freeze.
In addition to their environmental limitations, electro-chemical sensors have other potential limitations. For example, they are relatively slow in responding to changes in concentration of the gas being sensed. Also, they have a relatively short useful life.
Solid state sensors have been used as alternatives to electrochemical sensors. So-called metal oxide semiconductor (MOS) sensors can detect environmental hazards (e.g., hydrogen sulfide). An example of such an MOS sensor is Product No. S4000T (Intelligent Sensor) of General Monitor, 26776 Simpatica Circle, Lake Forest, Calif., USA. Another such sensor is an MOS sensor (Product No. 714) of Synkera, 2021 Miller Dr., Suite B., Longmont, Colo., USA.
Such MOS sensors have an electrical resistance which varies in response to the environmental hazard. For example, the sensor's resistance can vary in response to the concentration of hydrogen sulfide in ambient air. For example, by measuring this resistance, the amount of the sensed hazard can be computed. The resistance of an MOS sensor can be indirectly measured by measuring a voltage drop across the sensor (or current flow through the sensor).
Solid state sensors are not as susceptible to many of the environmental limitations of electro-chemical sensors (e.g., drying out in hot and dry environments). They also are rapid, durable and enjoy a relatively long operating life.
Notwithstanding their benefits, solid state sensors can present operational challenges of their own. For example, their accuracy can vary with changes in humidity and temperature.
To maintain accuracy, solid state sensors may be provided with an electrical resistance heating element to maintain the sensor at a desired operating temperature. For example, some MOS sensors perform with greatest accuracy when heated to about 300° C. Commonly, the resistive heating element is in direct contact with the solid state sensor for rapid conductive heating of the sensor by the heating element. The temperature of the sensor then can be indirectly determined by measuring the resistance of the heating element or the output of another temperature sensing device. Current to the heating element is altered to maintain the heating element (and, hence the sensor) at a desired operating temperature within tolerances which may vary from application to application.
While the foregoing maintains the sensor at desired operating temperatures when implemented in accordance with suggestions of the sensor's manufacturer, other problems arise. Namely, an undesired leakage of electrical current can develop between the sensor circuit and the heater circuit (e.g., through a heated dielectric or through impurities in the dielectric). The amount of such current is not easily determinable and can vary widely from application-to-application and over time. This leaked current can result in a false reading of the resistance of the sensor resulting in an erroneous interpretation of the degree of presence of the environment hazard of interest.